Cosmic Ray acceleration and Balmer emission from SNR 0509-67.5

TL;DR

This study models Balmer line emission from SNR 0509-67.5 to infer cosmic ray acceleration efficiency, considering shock dynamics, magnetic field amplification, and electron-proton thermalization, providing a method to estimate acceleration efficiency from observational data.

Contribution

We develop a semi-analytical non-linear shock acceleration model including neutrals, magnetic amplification, and shock dynamics to interpret Balmer line observations in SNR 0509-67.5.

Findings

Particle acceleration efficiency exceeds 30% for shock velocities above 4500 km/s.

The width of the broad Balmer line can be used to estimate cosmic ray acceleration efficiency.

Uncertainty in electron-ion thermalization affects the interpretation of Balmer line measurements.

Abstract

Context: Observation of Balmer lines from the region around the forward shock of supernova remnants may provide precious information on the shock dynamics and on the efficiency of particle acceleration at the shock. Aims: We calculate the Balmer line emission and the shape of the broad Balmer line for parameter values suitable for SNR 0509-67.5, as a function of the cosmic ray acceleration efficiency and of the level of thermal equilibration between electrons and protons behind the shock. This calculation aims at using the width of the broad Balmer line emission to infer the cosmic ray acceleration efficiency in this remnant. Methods: We use the recently developed non-linear theory of diffusive shock acceleration in the presence of neutrals. The semi-analytical approach that we developed includes a description of magnetic field amplification as due to resonant streaming instability, the dynamical reaction of both accelerated particles and turbulent magnetic field on the shock, and all channels of interaction between neutral atoms and background plasma that change the shock dynamics. Results: We achieve a quantitative assessment of the CR acceleration efficiency in SNR 0509-67.5 as a function of the shock velocity and different levels of electron-proton thermalization in the shock region. If the shock moves faster than ~4500 km/s, one can conclude that particle acceleration must be taking place with efficiency of several tens of percent. For lower shock velocity the evidence for particle acceleration becomes less clear because of the uncertainty in the electron-ion equilibration downstream. We also discuss the role of future measurements of the narrow Balmer line.